Adhesives used for optically clear display applications, e.g adhesives applied or laminated on a clear display substrate in an electronic assembly, must possess the properties of UV radiation resistance, thermal resistance, hydrolysis resistance, anti-yellowing and low haze. In addition, the photoinitiator which initiates a photo-curing reaction in the photo-curable adhesive composition under exposure to electromagnetic radiation, should possess good compatibility with the other components in the composition and have enough activity to initiate the radical polymerization. Currently, silicone polymers such as organopolysiloxanes are widely used as part of the photo-curable adhesive composition. However, commercially available photoinitiators are not sufficiently compatible or miscible with silicone system. In addition, many commercially available photoinitiators are not stable during or after the radical phot-curing reaction and cause problems such as haze and yellowness of the cured adhesive which are unacceptable especially in the optically clear display application.
U.S. Pat. No. 4,273,907 A discloses an organopolysiloxane modified benzoin based photoinitiator. Regarding practical applications, it has to be mentioned that such benzoin derivatives are only storable for limited time at ambient temperature. Such derivatives tend to be yellowing when exposed to thermal and/or UV radiation.
U.S. Pat. No. 4,536,265 A discloses a siloxane modified acetophenone based photoinitiator. Such acetophenone based photoinitiator also has tendency toward yellowing, and thus is not suitable for use in optically clear display application.
U.S. Pat. No. 5,776,658 A discloses a siloxane modified photoinitiator and photosensitive mixture. It is prepared from benzoin or α-hydroxy-alkylphenones with vinyltrimethoxysilane in the presence of RuH2(CO)(PPh3)3 as catalyst. It is reported that the photoinitiator and photosensitive mixture are hydrolytic stable. However, the Ru catalyst is expensive and vinylsiloxane alkylates the aromatic nucleus which is in ortho position to the carbonyl group. The para position is not occupied and could be oxidized, leading to yellowing when exposed to UV radiation.
EP 1072326 A2 discloses a siloxane-containing α-hydroxy-alkylphenones type photoinitiator. The photoinitiators reported in the patent application have a phenyl ether structure. Such structure is unstable under UV radiation and thermal conditions. The ether bond will degrade to phenol structure in harsh conditions and the phenol structure will finally transform to quinonoids which have deep colour and are unacceptable for use in an optically clear display application.
Therefore, it is the object of the present invention to develop an organosilicon modified photoinitiator which can overcome at least one of these challenges. These problems are solved by the disclosed photoinitiators.